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+#' Calculate the Principal Components
+#'
+#' @param data matrix, or dataframe with numerical data
+#' @param scalea logical value indicating whether the variables
+#' should be scaled to have unit variance.
+#' @param center a logical value indicating whether the variables
+#' should be shifted to be zero centered.
+#'
+#' @return prcomp object
+#'
+#' @examples
+#' calculatePCA(mtcars)
+calculatePCA <- function(data, scale = TRUE, center = TRUE) {
+  results <- prcomp(USArrests, scale. = scale, center = center)
+  #reverse the signs
+  # (eigenvectors in R point in the negative direction by default)
+  results$rotation <- -1*results$rotation
+  results$x <- -1*results$x
+  results
+}
+
+#' Calculate total variance explained by each principal component
+#'
+#' @param pca_results prcomp object
+#'
+#' @return vector with explained by each principal component
+#'
+#' @examples
+#' res <- prcomp(mtcars)
+#' get_total_explained_variance(res)
+get_total_explained_variance <- function(pca_results) {
+  x <- results$sdev^2
+  y <- sum(results$sdev^2)
+  return(x / y)
+}
+
+#' Create scree plot
+#'
+#' @param var_explained variance explained
+#'
+#' @examples
+#' plot_scree(c(0.8,0.1,0.005))
+plot_scree <- function(var_explained) {
+  nc <- length(var_explained)
+  ggplot2::qplot(c(1:nc), var_explained) +
+    geom_line() +
+    xlab("Principal Component") +
+    ylab("Variance Explained") +
+    ggtitle("Scree Plot") +
+    ylim(0, 1)
+}
+
+#' Test:
+#' head(USArrests)
+#' str(USArrests)
+#' pca_results <- calculatePCA(USArrests)
+#' biplot(pca_results, scale = 0)
+#' plot_scree(plot_scree)